Various strategies are being pursued to minimize the production and/or release of undesirable emissions from combustion processes. One such strategy is the development of technologies for the specific removal of acid gases from gas mixtures, such as the exhausts of carbon combustion processes. The separation of acid gases, such as CO2, from gas mixtures has been carried out industrially for over a hundred years, although no known process has been used on a large scale such as that required by large, industrial power plants. Of the numerous processes used for CO2 separation, current technology mainly focuses on the use of various solvents, such as alkali carbonates in the BENFIELD™ Process (UOP, LLC), alcoholamines in the ECONAMINE FG PLUS™ process (Fluor Corporation), and alcohols, diols, and ethers in the RECTISOL® process (Lurgi, GMBH) and the SELEXOL™ solvent (The Dow Chemical Company). In a typical solvent-based process, the gas mixture to be treated is passed through a liquid solvent that interacts with acidic compounds in the gas stream (e.g., CO2 and SO2) and separates them from non-acidic components. The liquid becomes rich in the acid-gas components, which are then removed under a different set of operating conditions so that the solvent can be recycled for additional acid-gas removal.
Methods for removal of the acid-gas components from rich solvents involve pressure and temperature change. Depending on the temperature of the gas mixture and the partial pressure of the acid-gas in the mixture, certain solvents are preferred for specific applications. When a solvent operates to interact with an acid-gas by chemical absorption, an exothermic chemical reaction occurs. The reversal of this reaction requires at least the amount of energy to be added back to the rich solvent that was produced by the forward reaction, not to mention the energy needed to bring the rich solvent to the temperature where reversal is appreciable and to maintain conditions to complete the reverse reaction to an appreciable extent. The energy required to obtain purified acid-gas from the rich solvent contributes to the cost of the purified product. In particular, the cost of the purified acid-gas has become a significant hurdle for the application of solvent technologies to fossil-fuel fired power plants for the removal of acid gases from flue gas.
Non-aqueous solvents have been used to remove CO2 from natural gas streams and require less energy for regeneration. Single-component alcoholic physisorption solvents such as RECTISOL™ and SELEXOL® are commercially available for CO2 separation but perform poorly in the humid, near-ambient pressure conditions associated with flue gas. Alcoholamines and amines have been combined with alcohols, diols, and cyclic carbonates by various researches to form “hybrid solvents” whose reaction mechanisms and kinetics have been studied in the literature. See, Alvarez-Fuster, et al., Chem. Eng. Sci. 1981, 36, 1513; Ali, et al., Separation and Purification Technology 2000, 18, 163; Usubharatana, et al., Energy Procedia 2009, 1, 95; and Park, et al., Sep. Sci. Technol. 2005, 40, 1885. In addition, a process known as the “phase-transitional absorption method” has been disclosed in relation to methods for deacidizing gaseous mixtures, which generally consists of the absorption of acid gases into an “absorbing phase” of less density than water consisting of a nitrogenous base and an alcohol, followed by transfer of the absorbed acid gas into an aqueous “carrier phase”. The aqueous carrier phase can be regenerated in a regenerator. The process claims to save energy by absorbing an acid gas at a faster rate than in an absorbing phase alone, and by avoiding the energy required to pump a rich absorbing phase to a separate regenerator by utilizing gravity to transfer the acid gas between phases in a single column for absorption and regeneration.
Ionic liquids are another non-aqueous solvent currently being developed. These solutions consist completely of ion pairs which are in the liquid state near room temperature. They have low regeneration requirements but have not surpassed aqueous amine solvents in performance due to factors including CO2 loading capacity, viscosity, cost, and, importantly, degradation by water. Using a non-aqueous liquid solvent to separate CO2 from gas mixtures containing water vapor can lead to the accumulation of H2O in the liquid solution either as a single-phase or bi-phase solution, depending upon the process conditions (e.g., pressure, temperature, H2O concentration) and the affinity of the non-aqueous solvent for H2O. H2O accumulation is detrimental to the CO2 separation and purification process, since more energy will be required for solvent regeneration due to the necessity of continually removing water from the solvent.
Another group of non-aqueous liquids which could be developed to address many of the problems affecting CO2 solvents are room temperature switchable ionic liquids. These equimolar mixtures of amidine or guanidine nitrogen bases and alcohols are non-ionic room temperature liquids that react with CO2 to form room-temperature ionic liquids. Typically, the conductivity of equimolar mixtures increases by one or two orders of magnitude when CO2 is added. Importantly, these solvents have higher CO2 loadings than some aqueous amines, and are regenerable under milder conditions. While these solvents are a promising alternative technology, those that have been previously disclosed are poorly suited for flue gas applications due to their chemistries with respect to water, which typically is a major component of flue gas. CO2 is captured via the formation of amidinium and guanidinium alkyl carbonate salts derived from the conjugate bases of the deprotonated alcohol components. However, if the conjugate base of the alcohol is a weaker acid than water, an acid-base equilibrium is established between the alcohol-conjugate base and water, which favors deprotonation of water and reformation of the protonated alcohol. The conjugate base of water, the hydroxide ion, reacts favorably with CO2 to form a bicarbonate anion, which requires more energy to reverse than alkyl carbonate anions.
Accordingly, it would be beneficial to formulate a new solvent system capable of effectively removing acid gases from gas streams (particularly water-containing gas streams) and which can be regenerated at a lower temperature and energy load than the solvents currently utilized for such purposes.